[tip: perf/urgent] perf/x86/intel/lbr: Zero the xstate buffer on allocation

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the perf/urgent branch of tip:

Commit-ID:     7f049fbdd57f6ea71dc741d903c19c73b2f70950
Gitweb:        https://git.kernel.org/tip/7f049fbdd57f6ea71dc741d903c19c73b2f70950
Author:        Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate:    Fri, 11 Jun 2021 15:03:16 +02:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Thu, 24 Jun 2021 08:49:03 +02:00

perf/x86/intel/lbr: Zero the xstate buffer on allocation

XRSTORS requires a valid xstate buffer to work correctly. XSAVES does not
guarantee to write a fully valid buffer according to the SDM:

  "XSAVES does not write to any parts of the XSAVE header other than the
   XSTATE_BV and XCOMP_BV fields."

XRSTORS triggers a #GP:

  "If bytes 63:16 of the XSAVE header are not all zero."

It's dubious at best how this can work at all when the buffer is not zeroed
before use.

Allocate the buffers with __GFP_ZERO to prevent XRSTORS failure.

Fixes: ce711ea3cab9 ("perf/x86/intel/lbr: Support XSAVES/XRSTORS for LBR context switch")
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Link: https://lore.kernel.org/r/87wnr0wo2z.ffs@xxxxxxxxxxxxxxxxxxxxxxx
---
 arch/x86/events/intel/lbr.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
index 4409d2c..e8453de 100644
--- a/arch/x86/events/intel/lbr.c
+++ b/arch/x86/events/intel/lbr.c
@@ -731,7 +731,8 @@ void reserve_lbr_buffers(void)
 		if (!kmem_cache || cpuc->lbr_xsave)
 			continue;
 
-		cpuc->lbr_xsave = kmem_cache_alloc_node(kmem_cache, GFP_KERNEL,
+		cpuc->lbr_xsave = kmem_cache_alloc_node(kmem_cache,
+							GFP_KERNEL | __GFP_ZERO,
 							cpu_to_node(cpu));
 	}
 }